


The First Spark

by Anchestor



Category: Undertale (Video Game)
Genre: Ecto-Genitalia (Undertale), Ecto-Vagina (Undertale), First Time, Fluff and Smut, Loss of Virginity, M/M, Smut
Language: English
Status: Completed
Published: 2018-09-15
Updated: 2018-09-15
Packaged: 2019-07-12 15:57:16
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,163
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/15998537
Author URL: https://archiveofourown.org/users/Anchestor/pseuds/Anchestor
Summary: After the hardship of war, monterkind has been sealed under the mountain. Grillby, however, has other things on his mind: the yearning ache in his Soul, the newfound, less than innocent desire for Gaster. A desire he’s not sure how to go about.It turns out Gaster is more than happy to be Grillby’s first.





	The First Spark

**Author's Note:**

> So, yeah. Happy Birthday Undertale, I guess. To celebrate have some smut?

Grillby was pacing nervously, treading a circular path to the ground near the campsite. It was a good spot to pace: far enough to feel secluded, near enough to be convenient. Grillby had a lot to nervously pace about these days, it felt like. The barrier sealing them under the mountain. What to do with himself now that the war was over. What this new life underground held in store. Not tonight though. Tonight, his worries were less about the uncertain future, and more about Gaster.

Gaster, and the pleasant, needy ache in Grillby’s Soul.

The feeling had crept upon him like the warmth and light of spring after a cold winter. It had started as little things. More blushing, getting more easily flustered. His kisses becoming bolder, hungrier. Not knowing how to hold himself while he waited for Gaster to get done bathing as Grillby sat by the riverbank.

And then, he slowly realized that simply kissing goodnight and falling asleep with Gaster in his arms didn’t feel like quite _enough_ any more.

It took longer still for Grillby to find a name for the strange, undirected energy buzzing inside of him. He longed for something, something that Gaster could give him, but he didn’t quite know what. But he yearned for it, _ached_ for it. Until one night, for no specific reason as far as Grillby could tell, stray pieces suddenly clicked into place. And Grillby had a word for the feeling.

Lust.

He was _lusting_ for Gaster.

For a while, that made the messy clash of emotions worse. He didn’t know what to think! What that meant! He had been summoned just a few years ago, and as such, Grillby didn’t know a lot about the world around him in the way that others did. The only thing he had ever been taught was how to fight. When it came to desire, the only schooling he had gotten was listening to other soldiers talk around the campfire, and that was completely mixed. Some spoke as if want was something shameful and embarrassing, shushing those who dared to speak up. Others bragged about their past conquests, like it was a display of some prowess. In the end, Grillby didn’t know what to think.

There was one person, of course, whom Grillby trusted to give him a straight answer. But the best person to ask was also the worst person to ask. _Gaster_.

Grillby swallowed. He loved Gaster. And Gaster loved him. And if it truly did turn out that Grillby was doing something wrong in his wanting, if Gaster thought him obscene… Grillby would simply have to trust that Gaster loved him enough to correct him and show him the right path. And, on the other hand, if Gaster returned his desire…

Well, Grillby wasn’t exactly sure what would happen then. But he’d have to trust that Gaster would tell him the right path in that case too.

No point in stalling. His conundrum as settled as it would ever be, Grillby headed back to the tent the two of them shared.

 

 

Gaster had already gotten ready for the night, discarding his heavy black robe, belt and boots. He was lying on his stomach on the bedroll, wearing only his undershirt and pants, the blanket pooled by his waist. He was reading some notebook, captivated by whatever it contained. For a moment, Grillby simply admired the sight of him. _By the stars_ Gaster was beautiful.

Gaster placed a mark between the pages, put the book away with a content sigh, and tilted his head to look at Grillby, greeting him with a smile.

“Good evening, darling. Ready for bed?”

“Yeah, just about.” Grillby sat on the edge of the bedroll, taking off his vest, boots and belt, neatly putting them away.

“Although… there- Uh. Um. There’s something I wanted to ask you about.”

He heard Gaster propping himself up, pushing the blanket aside, then shuffling to sit next to Grillby. The look on Gaster’s face was patient, encouraging.

Grillby opened his mouth, then closed it. He turned to look at the hands on his lap, suddenly at a complete loss for words.

“Is this about whatever has you so jumpy lately?” Gaster asked softly.

Grillby idly toyed with the hem of his tunic. “You noticed, huh?”

“Yeah, your flame has looked a bit jittery.”

Grillby swallowed, screwing his eyes shut. “I think I’d like to sleep with you.”

“…Grillby, we sleep together every night-”

“That’s not what I meant- I-”, Grillby interrupted. Gaster looked confused.

Slowly, carefully, Grillby reached for Gaster’s hands, holding them. Their fingers laced together, Grillby looked Gaster in the eyes.

“I’d like to… To lie with you.”

Gaster’s eye sockets went wide as he understood.

“Oh.” Was all he managed.

Then, a smile lit up his face. Gaster brought a hand to his face, and giggled.

“Oh, so that’s what you’ve been nervous about! You had me worried, I thought it was something bad!”

“…So it’s not a bad thing, then? To… To want someone like that?” Grillby asked tentatively, still unsure.

“No, of course not!” Gaster reassured. “Desire is completely natural. You just have to go about it the right way.”

An excited hope rose in Grillby, along a new kind of nervousness.

“So we could- I’d like to, tonight, but- Could you- It’s just-”, he floundered. “I’ve- I’ve never done this kind of thing before-”

“Grillby.” Gaster took Grillby’s hand, brought it to his face, and kissed it. “I would be honoured to be your first.”

Grillby blushed, giving a few sparks, and smiled. He let Gaster lead him as the two of them better settled on the bedroll, sitting facing each other in the middle.

“So, how am I supposed to…” Grillby gestured to fill the missing words.

Gaster brought his hand to Grillby’s face, gently caressing his cheek. “You’re not ‘supposed’ to anything. Just do whatever you think will feel good, however you think will feel good. And if at any point you feel uncomfortable, or want to stop, just say it.”

A small grin crept on Gaster’s face. “But if you want a suggestion, some kissing would be nice to start off, don’t you think?”

Eased by Gaster’s words, Grillby cupped the other’s face with his hands and kissed him. This, at least, was something he knew how to do.

The kiss was slow, familiar. Gaster brought a hand to Grillby’s neck, and with a hum of magic summoned his tongue. Grillby in turn pressed his own tongue against Gaster’s teeth, wordlessly asking for entrance. Gaster opened for him to slip in. Grillby explored the other’s mouth unhurried, licking along the inside of his teeth, feeling Gaster’s tongue as it prodded against his own.

The first time Gaster had summoned his tongue, Grillby had been a bit daunted by it. The purple ectoflesh had unnerved him. But once he, heh, _got a taste of it_ , he quickly learned that it made kissing a lot more fun.

As they kissed, Gaster’s other hand trailed along Grillby’s upper body, spending a few moments to tease the exposed skin by Grillby’s neck, meandering down, slipping under the fabric by the hem of the tunic, pushing it upwards.

“May I?” he asked in a husky voice, breaking the kiss.

Grillby nodded, and raised his arms as Gaster pulled the garment over his head. Gaster took a moment just to look at Grillby’s now bare chest, before letting his hand once again trail it, his touch feather light. Grillby gasped as Gaster hit a spot on his side, the simple touch so good it sent shivers up his back. Gaster grinned, prodding the spot he had found a bit further before continuing his exploration.

“Sometimes it feels so incredible that you just happen to look this beautiful. All chiselled and bright.” The touch of bony fingers on Grillby’s skin, slowly mapping his stomach, his sides, his chest, it felt _wonderful_. Grillby gave a contented sigh. Gaster tilted his head, as if pondering some mystery, and kissed Grillby again. Then he kissed the corner of Grillby’s mouth, the side of his chin, his neck, leaving a trail of little kisses all the way to Grillby’s shoulder. Never once did he stop his touches, searching for those good spots that made Grillby gasp. It felt nice, dizzyingly so.

“Hey, wait”, Grillby said, gently gripping Gaster’s wrist. He grinned shyly. “My turn now?”

Gaster nodded, and Grillby took the other’s shirt off. He had never really gotten to study Gaster’s bones the way Gaster had studied his body. Gaster was the doctor, and after the many times he had bandaged Grillby up he had become intimately familiar with his fiery build. Grillby, on the other hand, had only caught candid glimpses of Gaster’s body as he changed clothes or washed himself. Mimicking Gaster, Grillby let his fingers gently prod and stroke along Gaster’s ribcage, kissing his shoulder, leaving a path of kisses upwards like Gaster had done in reverse. As he reached the other’s neck, however, a choked moan escaped Gaster.

Grillby pulled back. Gaster covered his mouth with his hand, shocked by the sound he had made, a faint purple blush dusting his cheekbones.

“I- My spine is, uh. Sensitive.”

Grillby nodded, and brought a hand to one of Gaster’s bones, stroking along its length with his fingertips.

“What’s this bone called?” he asked.

“That’s my clavicle.”

“Clavicle.” Grillby bent forward to kiss it. “And this one?”

So they went over Gaster’s bones, one by one. Gaster named his sternum, his mandible, his true and false and floating ribs, and Grillby kissed or caressed each and every one.

“And this is your spine, I know that much at least”, Grillby said jokingly, trailing his pointer finger up and down it. Gaster gave a strangled gasp again. A sly grin spread on Grillby’s face. Soul fluttering, yet emboldened by Gaster’s reaction, Grillby wrapped his hand around the column of bone, carefully pumping it up and down, letting his fingers dig into the little nicks and grooves it had. This coaxed more of those wonderful sounds from Gaster, gasps and moans and whimpers alike. They made Grillby’s Soul flutter even harder, and he felt a soft, needy, pulsating sensation in his crotch, his pants feeling just a bit tighter than before. He tried to ignore it for now. Now was about making Gaster feel good.

Apparently what he was doing had an effect on Gaster, as a faint glow began to emanate inside his ribcage. With a familiar hum, magic pooled into Gaster’s pelvic area, solidifying into something still veiled by the fabric of his pants, glowing purple.

“Yeah, uh, my t-things work about the same way as my tongue”, Gaster stammered as a way of explanation.

“Can I see?” Grillby asked teasingly, lowering his hand to run his fingers along the edge of Gaster’s hipbone.

Gaster nodded, and after stealing one more quick kiss, Grillby gently pushed Gaster to lie down on the bedroll. Then, with some shuffling, he eased Gaster’s pants off.

However, what had formed between Gaster’s legs was not what he had expected.

“You… you have a…” Grillby was at loss of words.

Gaster was blushing harder by the minute, his face almost glowing with purple as he propped himself up on his elbows.

“S-so, since it’s all magic anyway, I can make pretty much whatever I want”, Gaster sputtered nervously. “And I thought you might like something, uh, more easily, um, p-penetrable?”

Grillby looked at the pussy made of ectoflesh. It looked a bit like a flower. …A boat-shaped flower.

“I-I can change it if it makes you feel weird!” Gaster sounded a bit panicked.

“What? No, no, it’s-” Grillby could feel his flame burning hotter as he blushed. “It’s lovely.”

“It’s just that I…” he continued. “I don’t really know how to… I mean, I’ve, um, touched a dick before, so I know the, uh, basics, but-”

Grillby shut his mouth, feeling disappointed in himself, his fire sinking low. He wished he had some way to research how to please someone, instead of blindly bumping into walls like this in the middle of the real deal. Gaster had to think he was a pathetic lover-

“Oh Grillby.” Gaster pushed himself halfway up, and brought a hand to touch Grillby’s face. “You’re such a sweetheart, you know that?”

Grillby sparked happily, his flame flickering high. Gaster took Grillby’s hand, bringing it between his legs, and pressing it against the mound there. He guided the movement, as he was touching himself with Grillby’s hand.

“You can rub it like that. There is a little nub, here-” He led Grillby’s fingers upwards. “It’s very sensitive, so it can feel really good, but don’t go too hard.”

“You feel… slick”, Grillby said, surprised. It was odd to feel something wet that didn’t hurt. It was similar how Gaster’s tongue felt: sort of wet, but not painful.

“That’s because I’m aroused”, Gaster said in a sultry voice. “You turn me on, Grillby.”

Gaster let go of Grillby’s hand, instead letting his fingers once again trail along Grillby’s upper body, bringing his other hand to Grillby’s neck. “You can put a finger or two in now.”

Grillby did, feeling along the folds of Gaster’s pussy at first, then pressing his pointer finger halfway in. He gently explored, massaging Gaster’s walls, soon to push in deeper, and adding a second finger. He made a point of irregularly grazing the nub. Gaster’s breathing became quicker, more laboured, Grillby’s prodding coaxing gasps and moans out of him once again. The throbbing need between Grillby’s legs became more intense, spurred on by the noises, his pants feeling ever tighter.

Gaster noticed his growing bulge.

“Well, well, well. What have we here?” He teased, palming Grillby’s clothed erection. He then slipped his hand in the waistband of his pants, helping Grillby to take them off as well as he could reach, finally freeing his hardened dick.

Gaster coyly examined it with his eyes as Grillby settled back between his legs. Then he grinned.

“Figures that you’d be _packing heat_.”

“The better to _bone_ you with”, Grillby retorted.

Gaster laughed. “Grillby, that was terrible!”

“Like yours was any better!” Grillby chuckled.

“Oh hush”, Gaster smirked, gripping Grillby’s dick and squeezing it gently. Grillby took a sharp breath. He hadn’t realized how badly his cock ached to be touched.

Gaster began to pump Grillby’s dick, stroking along its length, thumbing its tip. The movements had a certain confidence to them. An image of Gaster lying down, hand on his own cock flashed in Grillby’s head. Did Gaster think of Grillby as he touched himself, like Grillby thought of Gaster? Had he fantasised of what was happening at this very moment?

“Gaster-” Grillby was interrupted by his own moan as Gaster pressed a fingertip against his slit and smeared the precome beading there along Grillby’s dick.

“Yes?”

“Gaster, I, I want you, can, can I-” Grillby struggled to find the right words.

Gaster seemed to understand him nonetheless, letting go of Grillby’s dick, and spreading his legs further. “Just go slowly, okay?”

Grillby nodded. He aligned himself with Gaster, hovering over him, and began to push in, little by little. He sighed blissfully. The slick heat around him felt amazing-

“Grillby, wait, wait, stop-!” Gaster told him urgently, drawing in a hiss of breath, his hand quickly at Grillby’s hip to still him.

Grillby froze. “Need me to pull out?”

Gaster shook his head, his eyes squeezed closed.

“No, no, just, give me a minute.” He was breathing heavily. “Heh, you were a bit _more_ than I thought…”

As much as Grillby wanted to fully seat himself into Gaster, he waited. After a few moments, Gaster opened his eyes again, blinking at Grillby’s worried expression. He smiled.

“Hey now, don’t look so scared”, he said softly, touching Grillby’s cheek. “We’ll just have to remember to prepare a bit more next time.”

Grillby nodded, committing the words to memory.

“Okay, you can start going again.”

Grillby began to slowly push further in again, Gaster’s hand on his hip dictating the pace, until he had completely sheathed himself inside him. Gaster’s walls squeezed around him, his Soul beating so hard Grillby could almost hear it. But he wanted more, more of that amazing friction around him. Just as slowly as he had pushed in, he began to pull out, out, until only his tip was inside of Gaster. And then he pushed in again.

Gaster gave a breathy moan.

“Yeah, just like that…” he sighed happily, putting his arms around Grillby’s neck. “Keep going, just like that.”

Grillby set a gentle pace, carefully thrusting in and out. Gaster smiled up to him, sinking his fingers into the flame on top of Grillby’s head, and guiding him down for a sloppy kiss. The way their mouths met was messy, uncoordinated. Gaster’s other hand wandered along Grillby’s back, pulling him closer.

“Grillby”, Gaster said breathily. “Oh, Grillby, my darling Grillby...”

Grillby noticed that he was gradually increasing the pace, his movements growing faster, rougher. Gaster didn’t seem to mind though. Quite the contrary. He had begun to squirm and writhe under Grillby, bucking his hips to meet Grillby’s thrusts. His strangled gasps and sounds became louder, his hands grasping for Grillby as if he was a lifeline.

It all spurred the sensation building up inside of Grillby on. He couldn’t quite name it, it was like a coil wrung ever tighter, like a knot of a rapturous ache almost ready to burst, and it felt _amazing_.

“Grillby, Grillby-!” Gaster’s voice was needy, desperate. “Grillby, you’re going to make me-!”

Gaster’s skull rolled back, his mouth open in a silent scream, his walls fluttering around Grillby’s cock.

“Gaster-!” The name escaping him like a yearning plea, Grillby tipped over the edge. He was blinded by the feeling as he buried his head in the crook of Gaster’s neck, and for a moment, the world disappeared. No mountain, no tent, no up or down or left or right, nothing but their shared bliss. Together they rode through the wave of pleasure.

Grillby’s thrusts slowly grew tired, the high of the release winding down. He pulled out one final time, slumping on the bedroll next to Gaster. For a moment, both of them lied there, panting.

“So”, Grillby began, turning his head to look at Gaster. “Did I do alright?”

Gaster laughed breathlessly. “Alright?”

He rolled to his side, crawling into Grillby’s arms. They embraced, holding each other like they usually did when they slept together.

“Grillby, you were _wonderful_ ”, Gaster said, gazing lovingly into Grillby’s eyes.

Grillby smiled, and nuzzled Gaster’s face. “Good. I’m glad.”

Gaster reached for the blanket, and pulled it over the two of them, tucking them in.

The two of them stole kisses and cuddled, whispering sweetly to each other, until they both drifted off to sleep in each other’s arms.

**Author's Note:**

> Whether or not this was Gaster's first time too is up to you. Maybe he's on the ball about intercourse because he actually got pretty decent sex-ed, maybe he got busy with someone else. Who knows.
> 
> So, since this was baby's first time writing smut, feedback would be very much appreciated ^///^


End file.
